The present invention relates to a method of emitting navigation signals by a navigation transmitter as well as to a navigation satellite.
Satellite navigation systems, such as GPS, GLONASS and also the future European system with the project name Galileo, are sufficiently known from the prior art. Reference is made, for example, to J. P. Provenzano et al. “Implementation of a European Infrastructure for Satellite Navigation: From System Design to Advanced Technology Evaluation”, Int. J. Satell. Commun. 2000; 18:223-242.
Furthermore, systems for increasing the precision of the above-mentioned satellite navigation systems, such as the EGNOS (European Geostationary Navigation Overlay Service) System, are described in the prior art. Reference is made here, for example, to S. Badessi et al: “SNAV:SBAS Self-Standing Navigation Payload Based on Artemis Experience”, retrievable on the Internet under http://esamultimedia.esa.int/docs/egnos/estb/Publications/GNSS%201999/GNSS99_SNAV.pdf
FIG. 1 shows a payload conception corresponding to the EGNOS system with transparent transponders which are fed from a ground control station by utilizing an uplink. In this case, the ground control segment takes over the path measurements, the operation of the master clock (in the central master station) as the time reference, the synchronization of the master clock with the clocks in the ground control stations and the transmission of navigation signals to the EGNOS payload which the latter transmits largely transparently to the transmission devices of the communication payload of the EGNOS satellite. Particularly the time reference of this system is therefore situated outside the EGNOS satellite which acts as a navigation transmitter with respect to navigation transmit equipment.
FIG. 2 illustrates a payload conception with an onboard atomic clock as the time reference which is controlled only by way of the uplink. This type of conception is used, for example, in the case of the GPS and Galileo. However, this atomic clock is the most critical element of the navigation payload. It is sensitive and its life expectancy is limited. If this very sensitive atomic clock fails in the satellite, the satellite will be useless. However, the exchange of a complete satellite requires high costs and expenditures.
Similar problems may also occur in the case of other navigation transmitters, for example, fixedly or movably installed terrestrial navigation transmitters.
It is an object of the present invention to provide a possibility for reducing the susceptibility of navigation transmitters to interferences.
A first object of the present invention is a navigation satellite, having at least one receiving device for the receiving at least control signals and a transmitting device for sending navigation signals. The navigation satellite also includes a first signal path between the receiving device and the transmitting device which has at least the following:                A signal generating device for generating navigation signals and        a time reference device which is connected with the signal generating device.        
According to the invention, a second signal path is provided parallel to the first signal path while bypassing the signal generating device and the time reference device. The first signal path or the second signal path can optionally be connected by at least one switch-over device with the receiving device and/or the transmitting device.
As a result of these technical measures, an alternative interference-free signal path can be cleared in the event of an interference in the first signal path, particularly in the event of an interference in the signal generating device and/or in the time reference device. This alternative signal path bypasses at least the signal generating device and the time reference device. Particularly, in the case of an interference of the time reference device, this creates the possibility of feeding time reference signals from another time reference source to the transmitting device. The optional switchable connection of the first or second signal path can either take place only at the receiving device, only at the transmitting device or at the receiving device and the transmitting device.
A further development of the invention provides that the receiving device is connected by way of an identification-selective device with the first signal path and the second signal path, and the first signal path and the second signal path can be connected by way of a switch-over device optionally with the transmitting device. In this case, the first and the second signal path can always remain connected with the receiving device. Signals intended for the first signal path can then, corresponding to their identification, be fed to the first signal path, and signals intended for the second signal path, corresponding to their identification, can be fed to the second signal path. Any suitable physical or data-related description of the signals can be used as the identification, such as the frequency, the phase, the amplitude, the polarization direction, the modulation code (for example, in the case of CDMA), the bit structure, the header information of data packets, etc.
Thus, it can particularly be provided that the receiving device is connected by way of a frequency divider network with the first signal path and the second signal path. In this case, the frequency of the signal is therefore the identification according to which the signals can be selected corresponding to their destination for the first or second signal path.
However, as an alternative, it can also be provided that the receiving device is connected with the first signal path and the second signal path by way of a device for the selection of code-modulated signals. This measure can be selected when signals are transmitted in a CDMA (Code Division Multiple Access) process to the navigation satellite. In this case, the respective CDMA modulation code is used as the identification.
The second signal path can now be further developed such that it replaces interferences occurring in the first signal path and supplies corresponding replacement signals for signals which are subject to interferences or are absent in the first signal path. The second signal path may, in particular, have a signal processing function which differs from the first signal path. Thus, for example, the signal processing function of the second signal path may comprise only a frequency converter and/or a code converter. In this case, the replacement signals are not generated in the navigation transmitter itself but are received by way of the receiving device and are supplied by way of the identification-selective device to the second signal path which processes the signals only for a matching emission by way of the transmitting device.
Another object of the present invention is a method of emitting navigation signals by a navigation transmitter. In this case, a time reference is generated in a first signal path, and navigation signals are generated based on this time reference and are subsequently emitted by way of a transmitting device. According to the invention, in the event of interferences in the first signal path, a switch-over takes place to a second signal path parallel to the first signal path; the navigation transmitter receives time reference signals and/or navigation signals; and the received signals are guided by way of the second signal path to the transmitting device and are emitted.
Thus, when an interference occurs in the first signal path of the navigation transmitter so that time reference signals and/or navigation signals can no longer be generated or can only be generated in a faulty manner, a switch-over to the second signal path takes place, in which case, however, the time reference signals and/or the navigation signals are then received from the outside and are guided by way of the second signal path to the transmitting device and are emitted. This represents a very simple, fast and cost-effective measure for continuously maintaining the operation of the navigation transmitter despite an internal disturbance.
A further development of this method provides that, at least when there is no interference in the operation of the first signal path, at least control signals with a first identification are received by the navigation transmitter and, corresponding to their identification, are fed to the first signal path and, at least when there are interferences in the first signal path, time reference signals and/or navigation signals with a second identification are received by the navigation transmitter and, corresponding to their identification, are fed to the second signal path. The signals with the first and second identification can also be constantly transmitted to the navigation transmitter. However, preferably the time reference signals and/or the navigation signals will not be transmitted to the navigation transmitter before an interference has occurred in the first signal path. The distribution of the signals to the signal paths corresponding to their identification can take place by the above-mentioned identification-selective device. All above-mentioned types of identification can be used as the identification. Thus, a redundant source of time reference signals and/or navigation signals in addition to the internal source in the navigation transmitter can be created in a simple manner.
In particular, at least when there is no interference in the operation of the first signal path, at least control signals in a first frequency range are received by the navigation transmitter, are filtered according to their frequency and are fed to the first signal path, and, at least when there are interferences in the first signal path, time reference signals and/or navigation signals in a second frequency range are received by the navigation transmitter, are filtered according to their frequency, and are fed to the second signal path. Thus, the frequency or the frequency range in which the signals are transmitted to the navigation transmitter are used as the identification for the different received signals.
However, as an alternative, it may also be provided that, at least when there is no interference in the operation of the first signal path, at least control signals with a first code modulation are received by the navigation transmitter, are selected according to their modulation code, and are fed to the first signal path; and at least when interferences exist in the first signal path, time reference signals and/or navigation signals with a second code modulation are received by the navigation transmitter, are selected according to their modulation code, and are fed to the second signal path. This applies, for example, to the case of a CDMA modulation of the signals which are sent to the navigation transmitter.
Analogous to the above-mentioned statements concerning the navigation transmitter, it can also be provided here that the signal processing on the first signal path differs from the signal processing on the second signal path. Thus, it can be provided that, on the second signal path, a signal processing takes place only within the scope of a frequency conversion and/or a code conversion.
A further development of the invention provides that, at least in the event of interferences in the first signal path, time reference signals are received by the navigation transmitter which are emitted from a distant transmitting station with a time shift in such a manner that, after the reception and processing of the received time reference signal by the navigation transmitter, the time reference signal to be emitted by the navigation transmitter is synchronous with the undisturbed internal time reference of the navigation transmitter. Thus, the time reference signals from a distance transmitting station are provided with a time shift such that, after the transit through the signal route to the transmitting device of the navigation transmitter, the position of the time reference signals is identical with that which the internal time reference signals of the navigation transmitter should have in the undisturbed case. Thus, apparently nothing changes for a navigation receiver. It receives time reference signals which are absolutely identical and synchronous with the previously undisturbed internal time reference signals of the navigation transmitter.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.